1. Field of the Invention
This invention relates to an image forming apparatus such as a copying machine or a printer of the electrophotographic type or the electrostatic recording type in which a developer is caused to adhere to a latent image formed on an image bearing member to thereby make the latent image into a visible image.
2. Related Background Art
There is known an image forming apparatus in which predetermined image forming conditions, for example, a charging voltage, an exposure amount, developing bias, etc. are set to predetermined values and a patch-like image is formed on a photosensitive medium and the density thereof is read and the quantity of toner supplied is controlled on the basis of the result of this reading so as to keep the density of an output image constant.
As an example of the above-described image forming apparatus, a full color image forming apparatus is schematically shown in FIG. 8 of the accompanying drawings. In this image forming apparatus, a photosensitive drum 1 as an image bearing member is supported for rotation in the direction of arrow R1, and around it, there are disposed a corona charger 2, an optical system 3, a developing device 5, a transfer device 6 and a cleaning device 7.
The optical system 3 is a laser beam exposure device which comprises an original scanning portion and a color resolving filter and applies a color-resolved optical image E or an optical image E corresponding thereto to the photosensitive drum 1. The optical image E for each resolved color is applied to the photosensitive drum 1 uniformly charged by the charger 2 to thereby form an electrostatic latent image. The developing device 5 is a rotatable developing device comprising four developing devices, i.e., a black developing device 5BK, a cyan developing device 5C, a magenta developing device 5M and a yellow developing device 5Y, disposed around a center shaft 5a, and a predetermined one of the developing devices is rotated to a developing position opposed to the photosensitive drum 1 to thereby develop an electrostatic latent image on the photosensitive drum 1 and form a toner image on the photosensitive drum 1 using a toner having resin as a base substance.
Further, the toner image on the photosensitive drum 1 is transferred to a recording medium conveyed along a paper path indicated by a dotted line, from a recording medium cassette 30 through a conveying system and the transfer device 6, and supplied to a position opposed to the photosensitive drum 1. In the present example, the transfer device 6 has a transfer drum 6a, a transfer corona charger 6b, an adsorbing corona charger 6c for electrostatically adsorbing the recording medium, an adsorbing roller 6d opposed thereto, an inner corona charger 6e and an outer corona charger 6f, and a recording medium carrying sheet 6g formed of a dielectric material is cylindrically and integrally extended in a peripheral open region of the transfer drum 6a supported so as to be rotatively driven. As the transfer drum 6a is rotated in the direction of arrow R2, the toner image on the photosensitive drum 1 is transferred onto the recording medium carried on the recording medium carrying sheet 6g, by the transfer charger 6b. Toner images of other colors are successively transferred to the recording medium adsorbed and conveyed on the recording medium carrying sheet 6g and finally, a desired number of colored images are transferred to the recording medium, whereby a full color image is formed thereon.
When the transfer of a desired number of toner images is terminated in this manner, the recording medium is separated from the transfer drum 6a by a separating device 9 and is discharged onto a paper discharge tray 31 through a heat roller fixing device 10. On the other hand, any residual toner on the surface of the photosensitive drum 1 after the image transfer may be removed by the cleaning device 7 and the drum again becomes ready for use in a series of image forming processes.
The operation of controlling the density of the developer is performed in parallel with the above-described image forming processes. As shown in FIG. 2 of the accompanying drawings, a patch-like reference electrostatic latent image (hereinafter referred to as the "patch latent image") corresponding to a predetermined density is formed on the photosensitive drum 1 and it is developed by a toner and thereby made into a patch-like reference image (hereinafter referred to as the "patch image") P, and light is applied from an LED 11a of a density detection sensor (density detecting means) of the optical reflected light quantity detection type to the patch image P, and the reflected light therefrom is received by a photodiode 11b to thereby detect the density of the patch image P. Since this density corresponds to the toner density of the two-component developer in the developing device 5, the detected density is compared with a reference value to thereby take the difference therebetween, and on the basis of this difference, the amount of fluctuation of the density of the developer is calculated, and it is converted into a toner supply quantity (supply time), and a predetermined quantity of toner is supplied from a toner supply tank into the developing device 5, whereby control is effected so as to keep the density of the output image constant.
In the above-described prior art, however, the patch latent image formed on the image bearing member is developed into the patch image P, and the reflection density of this patch image P is detected by the density detection sensor and on the basis of the result of this detection, the supply of the toner is effected, but as shown in FIG. 3 of the accompanying drawings, there is eccentricity on the photosensitive drum 1 in each phase, and the gap between the photosensitive drum 1 and the developing sleeve of the developing device (hereinafter referred to as the "S-D gap") fluctuates by the rotation of the photosensitive drum and developing efficiency varies. FIG. 4 of the accompanying drawings shows the characteristic of the output of the density detection sensor for the S-D gap. As can be seen from this figure, the S-D gap fluctuates, whereby the density of the image varies and, as a matter of course, the output of the density detection sensor also greatly fluctuates. Therefore, it has been found that even if the patch image P is formed under the same image forming conditions, when the patch image formation position on the photosensitive drum 1 varies, the output of the density detection sensor includes not only the variation in the density of the developer, but also the variation in developing efficiency by the difference in the eccentric amount of the photosensitive drum, i.e., the difference in the S-D gap. Accordingly, if the output value of the sensor is simply fed back to the density control for the developer, then accurate control of the density of the developer can not be accomplished.